1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing planar fiber webs from short, oriented reinforcing fibers or fiber blends, wherein the reinforcing fibers or fiber blends are elutriated in a slurry liquid and subsequent to passing through a hydrodynamic orientation section, the fibers are deposited in an oriented condition on a filter surface while the slurry liquid is filtered off.
2. Discussion of the Prior Art
Methods are currently known for manufacturing planar performs or semifinishes, also referred to as fleece of fiber mats, of discontinuous (short), non-oriented reinforcing fibers. Their particular advantage, besides being inexpensive and ease of processing, lies in their uniform strength at all cutting angles. This strength, however, is in general rather low.
For the manufacture of high-strength components, the reinforcing fibers must be oriented in the form of continuous, endless individual filament strands (rovings), or layers (woven fabrics, roving bands, unidirectional roving prepregs, woven fabric prepregs), or in the form of unidirectionally oriented short fibers (UD short fibers) in the form of mats or mat prepregs. A prepreg is a semifinished article which is preimpregnated with a reactive resin blend and then predried.
The use of such UD short-fiber mats represents a compromise between high strength and good processability (ability to hug the contours of complex-surface molds, and so forth), between maximum utilization of the material and cost economy, and it facilitates the homogeneous, selective admixture of various types of fibers (socalled hybrid fiber materials).
Until the present, principally two methods for manufacturing UD short fiber mats have become known: the ERDE process (Explosives Research and Development Establishment, Waltham Abbey, UK; Lit.: Dingle, L.E., Conf. Carbon Fibres, London, February 1974, the Plastics INstitute ISBN 090310704X Paper No. 11); and the socalled MBB vacuum barrel filter process (German Patent Specification 2 163 799; Richter, H., Kunststoffe 67 (1977) 12, p. 739). Both methods have the common feature in that the short reinforcement fibers or fiber blends are converted into a slurry by employing a suitable liquid, preferably glycerine, and after passing through a hydrodynamically operating orientation section, they are deposited in an oriented condition. In the ERDE process this is achieved through orienting within a single moving nozzle and deposition on plane filter panels, whereas in the MBB vacuum barrel filter process this is achieved by means of a wide pouring trough which is formed by a separate component and has guide ducts running in parallel with the direction of the slope and with the deposition of the fibers on the exterior of a horizontally rotating filter barrel, with the slurry liquid being aspirated into the interior of the barrel under the effect of a vacuum.
In both of the prior art methods the filter cake, after deposition, must be carefully washed several times in order to remove the glycerine, which is incompatible with the subsequent resin impregnating process. Hereby the orientation of the fibers may again be disturbed. The two methods also have in common that they operate very slowly because, initially, the slurry liquid must flow laminarly in the nozzles or ducts so as to effect the orientation of the fibers and, secondly, both methods operate only a single deposition nozzle or trough. Moreover, both methods operate principally discontinuously.